1. Field of the Invention
The invention relates to a method for controlling the passage of rolled stock under little longitudinal tensile stress through a continuous rolling mill by secondary-controlled hydrostatic drives for the individual rolling stands, the rotary speeds of the hydromotors associated with the drives being controlled by adjusting their displacement volume.
2. Description of the Prior Art
Several rolling stands are arranged sequentially in a continuous rolling mill, in which slabs, billets or the like are rolled to obtain a desired gage by successively reducing the cross section of the material by a predetermined amount. Because of the condition of continuity of the passage of the rolled stock through the rolling mill, the product of the material cross section and the rolled stock speed is constant at each point and, therefore, the rotary speed of the rolls must increase substantially proportionally to the decrease in the cross section from stand to stand. Since it is desired to obtain a rolled stock passage which is as free as possible from tensile and compression forces and no method has been available heretofore to permit the required rotary speeds of the rolls to be calculated beforehand, a good rolling result depends on a suitable rotary speed control, it being impossible, however, to measure the tensile and pressure forces prevailing in the rolled stock wherefore they cannot be used as guiding parameters for the control of the rotary speed.
Various control methods are known for rolling mills with direct current motors for the rolling stand drives, which operate, for example, during the passing stage with a guiding stand operated at constant speed and with individually adjustable rotary speed governors at the other stands (EP-A1000 8037) or in which the rotary speed control circuit of each stand through which the stock passes is separated and this stand is controlled until the stock passes through the next stand by a subsidiary torque control circuit in dependence of a change in the drive torque of the preceding stand (DE-OS 2 413 492). However, this has the fundamental disadvantages of an electrical drive and the high costs of the computer circuits and the measuring devices.
In contrast to the electrical drives, hydrostatic drives for continuous rolling mills have found favor because of their compact construction, their high efficiency and their low inertia, secondary-controlled drives being usually preferred. In a secondary-controlled hydrostatic drive, a common pressure medium system comprised of pumps and hydro-accumulators supplies a constant pressure to the hydromotors so that it is not necessary to provide a separate pumping station for each hydromotor and a considerable saving may be obtained in driving power. In these secondary-controlled drives, the displacement volume of the hydromotor may be steplessly adjusted by an adjustment device from zero in both directions and the rotary speed of the motor may be controlled by the change in the displacement volume. If the prevailing rotary speed is compared to a predetermined desired rotary speed by a control device and the control device is controlled in dependence on the comparison between the prevailing and desired values, the load may be held to the desired rotary speed when the drive torque changes. Because of the supplied pressure at the inlet of the motor, the displacement volume of the hydromotor is proportional to resulting torque of rotation at a predetermined rotary speed so that a specific displacement volume is produced at the motor at a predetermined rotary speed and a given torque of rotation. To prevent a substantial break in the rotary speed when the torque of rotation is suddenly changed, pressure relief valves or like devices are built into the pressure supply line for the hydromotors to prepare the motor for the sudden change of the torque of rotation by properly changing the pressure supplied to the motor inlet and the proportional change in the displacement volume. This, however, has basically no significance for the rotary speed control proper.
In the control of the rolled stock passage through a continuous rolling mill with secondary-controlled hydrostatic drives, it is known from AT-S 383 059 to make use of the position changes of the adjustment devices of the individual hydromotors for correcting the rotary speed of the hydromotors for the adjacent rolling stands to hold the tensile forces during the passage of the rolled stock as low as possible. This control method, however, also requires high-cost measuring techniques for determining the position changes of the adjustment devices and also high-cost computer techniques for calculating the required rotary speed corrections for the adjacent rolling stands.